Polarization mode dispersion (PMD) is a distortion mechanism, like chromatic dispersion, that causes optical devices, such as single-mode fibers, optical switches and optical isolators, to distort transmitted light signals, which can result in random signal fading, increased composite second order distortion, and increased error rates. The relative severity of PMD, which is a function of the wavelength of the transmitted light, has increased as techniques for dealing with chromatic dispersion have improved, transmission distances have increased, and bit rates have increased.
PMD is due to differential group delay (DGD) caused by geometrical irregularities and other sources of birefringence in the transmission path of the optical device. For example, a single-mode fiber (SMF) is ideally a homogeneous medium supporting only one mode. In practice, it supports two propagation modes with orthogonal polarizations. When a lightwave source transmits a pulse into a SMF fiber, the pulse energy is resolved onto the principal states of polarization (PSP) of the fiber. The two groups of pulse energy propagate at different velocities and arrive at different times causing pulse broadening and signal distortion. When the core of the fiber that bounds the light is asymmetrical, the light traveling along one polarization axis moves slower than the light polarized along the other axis, which can result in the pulse spreading, thereby overlapping with other pulses, or changing shape, thereby making it undetectable at the receiver.
Determining the PMD of installed optical fibers is useful for determining the capacity of the fibers for transmitting new telecommunication services, and for the design and control of PMD compensators.
The PMD of a fiber is commonly characterized by two specific orthogonal states of polarization called the principal states of polarization (PSPs) and the differential group delay (DGD) therebetween. The DGD can be described at an optical wavelength λ by the 3-component Stokes vector, [Ω(λ)]=Δτq, where q is a unit Stokes vector pointing in the direction of the faster PSP, and the magnitude Δτ is the DGD. Typical DGD values encountered in transmission systems range between a few tens of fs and 100 ps.
Known methods for determining PMD vectors include the Jones Matrix Eigenanalysis (JME) technique and the Muller Matrix Method (MMM). Each of these techniques uses a tunable, continuous-wave laser and a polarimeter to measure the output polarization states for two (or three) different input polarization launches at two optical frequencies. The PMD vector is then calculated for the midpoint frequency. In addition to determining the output PMD vector, the Muller Matrix Method determines the rotation matrix of the fiber at each frequency and thus the input PMD vector can be calculated. Shortcomings of these techniques are that they are somewhat difficult to implement, particularly in a feedback system, because they require frequency differentiation of measured data at plural optical frequencies.
The disclosed invention relates to the measurement of polarization mode dispersion with a Fixed Analyzer-Fourier Transform (FA-FT) method or an interferometric (INTFER) method, which are well known in the prior art. Typically, for the INTFER method, a broadband light source is sent through a DUT and then into an interferometer. As the moveable arm of the interferometer is reciprocated, interference fringes are observed at a detector only if the time-delay difference between the two arms matches a delay generated in the DUT to within the coherence time of the light source. For a non-mode-coupled device, a delay histogram with a central peak and two side lobes is formed by plotting the envelope of interference fringes as the moveable arm of the interferometer is scanned. The central peak is the autocorrelation of the source, which provides no information relating to the PMD; however, the distance from either side lobe to the central peak is a measure of the average DGD over the spectrum of the light source. Alternatively, the separation between the two side lobes is equal to twice the average DGD over the spectrum of the light source.
The INTFER method can also be used to measure the DGD in mode-coupled devices; however, if there are N mode coupling sites, there will be 2N+2−1 peaks in the resulting delay histogram. The separation of adjacent peaks can easily be less that the coherence time of the light source, and so the peaks are not necessarily distinguishable; accordingly, the resulting delay histogram envelope comes from the coherent addition of the various delays. The RMS DGD value can be obtained from the second moment of the “Gaussian-shaped” delay histogram. Unfortunately, there are non-ideal features of the delay histogram that make it deviate from a true Gaussian shape, e.g. the autocorrelation peak and the noise floor. Attempts have been made to correct for these features during the mathematical calculation, but so far none have been completely successful.
The FA approach, a.k.a. the wavelength scanning approach, indirectly measures the mean DGD by detecting light transmitted through a polarizer/DUT/polarizer set-up as a function of wavelength. The light source can either be a tunable laser, requiring a single detector, or a broadband source with an optical spectrum analyser. As the output polarization vector sout(ω) moves around the Poincare sphere, the normalized intensity IN(ω) transmitted through the output polarizer can be defined as:IN(ω)=½(1+sin Φ cos [θ(ω)]sin φ+cos Φ cos φ)
In which the angles are in Poincare sphere coordinates, Φ is the angle between Ω and the Stokes vector describing the transmission axis of the output polarizer, and φ is the angle between sout and Ω. φ and Φ are independent of ω for non-mode-coupled devices. θ(ω) is the aximuthal angle of the precession of sout (ω) about Ω. For non-mode-coupled devices, θ(ω) depends approximately linearly on c and contains all of the optical frequency dependence of IN(ω). Accordingly, dθ/dω and thereby the average DGD (Δτ) can be determined by counting the number of extremer, i.e. peaks and valleys, in the sinusoidal IN(ω) curve over a given optical frequency range. Alternatively, the IN(ω) spectrum can be Fourier transformed (FA-FT) into the time domain, resulting in a delay histogram very similar to one, which would be produced from an interferometric measurement utilizing the same optical light source spectrum.
However, the DUT always exhibits eigenstates of polarisation with a slow and fast axis, which is true even for the most common case of strong coupling fibres, although in that case the eigenaxes rotate with the wavelength. Considering that the light is launched without any control on the orientation of its polarization state with respect to the fibre eigenaxis at the input, nor on the orientation of the output polarizer with respect to the fibre eigenaxis at the output, it can easily be understood that the measurement suffers from poor repeatability. The repeatability is directly related to the number of rotations of the eigenaxis over the wavelength span of the analysis. More precisely, the standard deviation of the measurements is inversely proportional to the square root of the product of the wavelength span and the PMD value of the DUT, which was demonstrated by Gisin et al in “How accurately can one measure a statistical quantity like polarization-mode dispersion?”, IEEE Photonics Technology Letters 8, no. 12, p. 1671 (1996) and applied to a set of different wavelength spans for a given DUT. Unfortunately, Gisin et al did not consider different launch conditions; however, experiments by the inventors of the present invention showed that their formula fit a set of different launch polarisation states for a given DUT and wavelength span. With commonly available sources that can cover a spectral range of about 100 nm to 150 nm and a DUT with a rather large mean PMD, e.g. 10 ps, the standard deviation of the measurement is about 3% of the value measured. Given the inherent variations of the PMD value with temperature and strain, 3% is considered by most as a good enough accuracy for determining whether or not a fibre may carry 10 Gbit traffic. When considering a 2.5 ps mean delay, which is the limitation for 40 GBit traffic, the standard deviation rises by a factor 2. The purpose of the present invention is to recover this accuracy degradation and offer a measurement at 2.5 ps that is as accurate as the current devices are at 10 ps. Note: in presence of attenuation or in the case of amplified links, the available spectral range is lowered (down to about 35 nm for a C-Band amplified link) with a consequent diminution of the accuracy.
In the past, one possible solution described in literature and standards is to repeat and average successive measurements with different input/output. However, polarization controller/scramblers are quite costly devices, and the repetitive process increases the duration of the measurement. Accordingly, a passive device to do the scrambling in another manner and yield a proper repeatability in a single measurement is more desirable.
The use of a birefringent or wavelength specific “artefact” was proposed in U.S. Pat. No. 5,654,793 issued Aug. 5, 1997 to Barlow et al, in order to bias the PMD away from the “spurious (near zero) PMD response”, and enable measurements below 0.1 ps. Unfortunately, the Barlow et al method does not improve the repeatability of the measurement because for such small DGD the spread depends strongly on the polarization mode coupling between the fibre under test (FUT) and the highly birefringent (HiBi) fibre. With a single birefringent element inserted in the optical path, two eigenaxis are privileged, and the measurement remains strongly polarization sensitive, in particular for PMD below 0.1 ps for which even a strong-coupled fibre exhibits weak coupling features, which was demonstrated by Oberson et al in “Interferometric PMD measurements with femtosecond sensitivity”, J. Lightwave Techno. 15, No. 10, 1997, who proposed to monitor the bias frequency while actively controlling the polarisation launch conditions and successfully measured very low PMD values. Unfortunately, Oberson et al method required active control over the launch conditions, while, as already mentioned above, a method that uses a passive device is more desirable.
An object of the present invention is to improve the accuracy and repeatability of the measurement over the whole measurement range. Moreover, working with fast “ac” signals will help to isolate the signal of interest out of the amplifier's ASE and slow variations.